Thermocouples are temperature sensors consisting of two conductors that are joined together at a junction and that have two free ends spaced apart from the junction. The conductors are made of different metals such that when a temperature gradient is applied between the free ends and the junction, a voltage appears between the free ends. The size of the voltage corresponds to the size of the temperature gradient. As a result, using the voltage and the temperature at the free ends, it is possible to determine the temperature at the junction.
If the conductors of the thermocouple come in contact with each other between the free ends and the junction, they create a short in the thermocouple that produces a new junction. When this occurs, the voltage produced at the free ends of the thermocouple corresponds to the temperature at the new junction created by the short and not at the original junction of the thermocouple. Because of this, temperature calculations produced from the voltage will not accurately reflect the temperature at the original junction if there is a temperature differential between the location of the short and the original junction.
In process control environments, thermocouples are often used to measure the temperature of a process fluid since they can withstand a wide range of process temperatures. Typically, the free ends of the thermocouple are connected to the interior of a temperature transmitter, which measures the voltage between the free ends of the thermocouple, measures the temperature within the transmitter at the free ends, and uses the temperature in the transmitter and the voltage across the thermocouple to calculate the temperature at the junction. This junction temperature is then communicated to one or more other process devices located in the process environment or to a control room either over a wired or wireless connection.